Decorative laminates are widely used as surfacing materials for walls, countertops, furniture and other structures, such as aircraft interior panels. Decorative laminates are used in such structures because they can simulate the appearance of many materials, such as wood, and because laminates are capable of being molded and embossed to provide a three-dimensional surface. Decorative laminates are formed by a plurality of layers that include a substrate layer, a decorative layer that overlies the substrate layer and a protective layer that overlies the decorative layer. Other layers may be included between the substrate, decorative and protective layers in specific decorative laminates, depending upon the nature of the materials used and the desired laminate characteristics, e.g., flat or embossed. Laminates that are flat or have a single contour are known as "rigid laminates." Laminates with two or more contours are known as "flexible laminates." Both rigid and flexible laminates are embossable.
In recent years, the substrate layers of aircraft interior panel rigid decorative laminates have been formed of epoxy resin composites. Structurally, such rigid panels include decorative ink patterns or decorative sheets (e.g., a decorative layer) located between an epoxy resin composite substrate layer and a protective layer formed of a suitably transparent material such as polyvinyl fluoride. The substrate layers of flexible laminates have been formed of embossable films which have high smoke and heat release characteristics due to the nature of the resins used (e.g., polyesters). Structurally, the flexible panels are equivalent to the rigid panels with the exception of the substrate layer.
Decorative laminates that include epoxy resin composite and embossable film substrates are advantageous because they are economical both in terms of material cost and processing complexity. Such decorative laminates have the further advantage that they maintain a high decorative quality during the lamination process, partially due to the fact that reaction products, created when the epoxy resin in the substrate layer is cured, are minimal and do not have a disrupting effect on either the decorative pattern or the outer protective film. The high decorative quality of decorative laminates and their conformance to previously existing safety standards attributed to their past popular use in the industry.
Recently, the Federal Aviation Administration (FAA) has amended the Federal Aviation Regulations (FAR) to increase the certification requirements for compartment interiors for carriers with at least a 20 passenger capacity. Two FAR sections have been amended to tighten the heat release standards for interior components, such as partition, ceiling and wall panels, and the outer surfaces of galleys, large cabinets, and stowage compartments.
The new FAA certification requirements upgrade the heat release over a period of time and peak rate heat release criteria used to evaluate materials to be used in the interior of aircraft. A sample piece of a component, with laminate attached, generally 150 mm.sup.2, is hung vertically and its decorative surface is exposed to a heat release apparatus that emulates the radiant heat present in a post-crash environment. Each sample piece is tested three times for five minutes. This is referred to as the Ohio State University ("OSU") test. The acceptance criteria have been changed from a total release of 100 kilowatt-minute per square meter over the first two minutes to 65 kilowatt-minute per square meter, and from a peak rate heat release of 100 kilowatts per square meter to 65 kilowatts per square meter. The peak level was adopted to prevent the use of materials which have relatively low levels of total heat release but which emit a large amount of heat over a short duration. The FAA asserts that inherent in these heat release standards are limitations on smoke and toxic emissions due to the significant known correlation between those characteristics and flammability characteristics. One aspect of the correlation between toxic emissions and flammability in closed areas relates to the onset of "flashover." Flashover is the combustion of high levels of combustible toxic emissions within an enclosed or partially enclosed area. Thus, an increase in flammability requirements implies a reduction in toxic emissions and a delay in the onset of flashover.
Epoxy resin based laminates were subjected to testing during the FAA rulemaking period. It was determined that the continued use of existing epoxy or polyester resin based laminates was undesirable because, when heated, such laminates do not meet the stricter heat release criteria. Another previously known disadvantage of epoxy resin based laminates not addressed by the FAA is their emission of large amounts of smoke and toxic gas during heat stress. Even though this aspect of epoxy resin based laminates was not addressed by the FAA, it is desirable that any solution to the epoxy resin heat release failure problem also address the smoke and toxic gas problem.
In addition to testing epoxy resin based laminates during its rule making process, the FAA also tested phenolic resin based laminates. The results of the tests led the FAA to the conclusion that certain phenolic resin based laminates would meet the new standards. Because ways of creating phenolic resin composite substrates were known in the heat and pressure laminate art, the only change to existing laminate producing methods foreseen was a change in the type of resin used in the substrate layer. No major problems in the relationships between the resin and other laminate materials were expected. (An additional advantage not noted by the FAA is that phenolic resin based laminates do not emit large amounts of smoke and toxic gas when heated.) Unfortunately, the FAA's presumption regarding the ability to produce commercially acceptable decorative laminates with phenolic resin based substrates using existing methods has proven to be incorrect. During curing, phenolic resins release a large amount of condensation reaction products such as water and formaldehyde, in addition to a residual quantity of free phenols. These reaction products attack decorative ink pigments, causing the ink to discolor, resulting in the deterioration of their decorative quality. Changes in the type of phenolic resin only affect the degree of deterioration, not the occurrence of deterioration. Thus, while the phenolic resin based laminates may meet the new FAA heat release standards, commercially acceptable decorative laminates that include a phenolic resin based substrate cannot be created by simply substituting a phenolic resin for an epoxy resin in existing methods of creating decorative laminates.
In summary, prior epoxy resin based decorative laminates no longer comply with the FAA heat release standards. While phenolic resin based decorative laminates created in the same manner as prior epoxy resin based decorative laminates meet the new FAA heat release standards, such phenolic resin based decorative laminates are commercially unacceptable because when phenolic resins are cured, they produce reaction products that discolor the ink used to create decorative patterns and, thus, destroy the decorative patterns. This invention is directed to providing high quality (e.g., commercially acceptable) decorative laminates that meet the new FAA heat release standards.